Integrated circuitry typically comprises electrical interconnections. A typical semiconductor electrical connection includes a metal or other conductive layer communicating with other layers located at different elevations within a substrate. Forming electrical interconnections is typically conducted, in part, by etching an opening through insulating material, and then flowing metal or other conductive material to within the opening.
An exemplary semiconductor structure comprises a bulk monocrystalline silicon substrate with an overlying silicon dioxide layer as the insulating material, such as borophosphosilicate glass (BPSG). BPSG can be considered to be silicon dioxide doped with phosphorus and boron. An opening can be formed through the BPSG layer and to the underlying substrate with a suitable etch. After etching through the BPSG layer to form the opening, the silicon substrate is exposed to the environment, routinely including oxygen. Silicon reacts with oxygen to form an oxide, specifically silicon dioxide, a material having insulative properties. Accordingly, a film of silicon dioxide, commonly referred to as native oxide, forms on the exposed silicon. If the native oxide is not removed, the conductive properties of an electrical connection formed within the opening can be diminished.
Typically, the native oxide is removed by an etching process, preferably an oxide etch. However, because the BPSG layer is also an oxide and defines the sidewalls of the opening, the sidewalls will etch during removal of the native oxide. The etching of the sidewalls can widen the opening. Moreover, because doped materials etch faster than less doped materials, and because BPSG is essentially a doped oxide, the sidewalls will etch faster than the native oxide. This etch rate differential increases the difficulty to fabricate and control the width dimensions of a opening. With the emphasis in the semiconductor industry to increase the density of active components per unit area of semiconductor substrate, it is typically desired to keep openings narrow. It would, therefore, be desirable to develop improved methods of forming openings in doped substrates.